There exist several new processes, which aim at recover the nutrients, mainly phosphorus from sewage sludge. ASH DEC for instance refers to N-P-K fertiliser, which is produced based on a sewage sludge ash containing a lot of phosphorus enriched with other nutrients such as nitrogen, potassium or sulphur by using the patented ASH DEC technology. The thermo-chemical process has undergone extensive testing in a semi-industrial pilot plant since 2008 and has now achieved industrial maturity. The produced phosphate fertilisers contains significantly lower levels of contaminants than conventional mineral fertilisers and has proven to be even more effective in terms of dry mass yield and plant uptake in some soils. A similar product is called Crystal Green. It can be produced in a unit directly integrated in a wastewater treatment facility and produces a solid granular fertiliser.

As fertiliser prices raise and phosphorus resources decline, the recover of nutrients, mainly phosphorus from wastewater and sewage sludge sees increasing interest (see also peak phosphorous). Today, there exists already several processes, at small or large scale to recover these nutrients the most often in form of struvite, also called Magnesium Ammonium Phosphate Hexahydrate (MAP) (see also struvite and struvite precipitation from urine). The production and use of such local and renewable source for fertiliser can help to optimise the water and nutrient cycle.

The patented ASH DEC technology of the ASH DEC Umwelt AG (Austria) is one of many ways to regain nutrients lost in incineration sewage sludge and to make them available to agriculture. In comparison to other processes, the ASH DEC process has been sufficiently researched so that it is now applicable at industrial scale.

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The ASH DEC fertiliser and process

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The thermo-chemical process removes the toxic heavy metals from the ash and induces chemical nutrient compounds that plants can easily take up from the soils. With its process, ASH DEC produces a multi-nutrient fertiliser out of sewage sludge ash, which meets all relevant parameters of national regulations in Europe. With regard to its cadmium concentrations, it beats mineral phosphate fertilisers by one order of magnitude. During the process, toxic substances are evaporated and concentrated in the flue gas cleaning system, whereas the nutrients become plant-available. The process recovers primary (P, K), secondary (Ca) and trace nutrients (Mg, Fe, S). Particularly P reaches concentrations comparable to standard single phosphate fertilisers.

Then, ash and harmless chlorine donors are mixed and compacted in a pellet press. Reagents are dosed in accordance with ash quality as well as legal and final-product requirements.

The pellets are fed to a thermal reactor and exposed for about 20 minutes to a temperature of 1.000° C. At this temperature, up to 99% of the target metals - usually mercury, cadmium, lead, zinc and copper - react with the additives and evaporate.

98% of the input ash arrives in form of P-rich granules without toxic substances. 2% of the input ash is retained in the air pollution control system in form of a metal concentrate.

To produce fertilisers that meet the requirements of soils and crops, the P-raw product is enriched with other nutrients such as nitrogen, potassium, sulphur or an additional portion of phosphorus.

ASH DEC process. Source: ASH DEC (2008)

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Crystal Green (Ostara Inc.)

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(Adapted from OSTARA 2011)

Crystal Green refers to NPK fertiliser produced from the effluent of wastewater treatment plants in a process called Pearl®. The process can be integrated directly into a treatment system and helps also to prevent unwanted precipitation in pipes etc. It is based on controlled chemical precipitation in a fluidised bed reactor that recovers struvite in the form of highly pure crystalline pellets. The nutrient-rich feed streams (liquid fractions of sludge are first mixed with magnesium chloride and, if necessary, sodium hydroxide and then fed into the reactor where the struvite “seeds” e begin to form. These seeds grow in diameter until they reach the desired size of about 1-3.5 mm. In a municipal wastewater treatment plant, up to 90 % of the phosphorus and 40 % of the ammonia load is removed from sludge dewatering liquid using this process and the resulting product is marketed as a commercial fertiliser called Crystal Green®. The fertiliser is adaptable to any types of turf, field-grown nursery stock and high-demand phosphorous crops. Because this fertiliser is solid, the risk of leaching or runoff is significantly reduced.

Crystal Green is a white fertiliser in a granular form. Source: OSTARA (2011)

Applicability

In general, incinerators that combust sewage sludge, meat and bone meal, digestion residues from biogas facilities and similar material with usually no or little co-combustion of industrial residues qualify for nutrient recovery to produce fertilisers. Preferred plant locations are close to incinerators of reasonable combustion capacity.

Yet, it must be mentioned that this technology is very high-tech and highly energy intensive. It may be a good technology in highly industrialised countries where laws prohibit the use of treated sewage sludge in agriculture — it can then help to close the nutrient loop. Yet, there are other methodologies that allow for a much simpler and less energy intensive nutrient recovery than for instance burning sludge (that usually has a very high moisture content). The technology requires significant financial investment, though ASH TEC is willing to contribute to this.

This presentation describes the need for recycling phosphorus and the economic sector of the enterprise. There is further information on the process and the pilot plant. Aspects of benchmark and finance are included as well as a brief description of the need for recycling phosphorous (in German).

This short information paper describes the development of ASH DEC Umwelt AG. The process and the product are described shortly and a few historical key figures of the enterprise’s development are mentioned (in German).

The present study provides an overall perspective on technologies for the recovery of phosphorus from wastewater treatment plants and the opportunities they offer. It differs from previous work in that it not only provides technical process details, but also – and particularly – highlights the status of development, probable availability, and economic viability of the processes, providing the reader with an aid in decision making.

This short information paper describes the development of ASH DEC Umwelt AG. The process and the product are described shortly and a few historical key figures of the enterprise’s development are mentioned (in German).

This presentation by ASH DEC Umwelt AG describes the need for recycling phosphorous and the economic sector of the enterprise. There is further information on the process and the pilot plant. Finally, ASH DEC gives general information on their fertiliser product and explains their idea of joint operating company (in German).

The presentation gives an overview on global trends in terms of food consumption, fertiliser needs and consumption in the agricultural sector, fertiliser prices and the potential of ‘mining’ urban phosphorus resources.

This presentation describes the need for recycling phosphorus and the economic sector of the enterprise. There is further information on the process and the pilot plant. Aspects of benchmark and finance are included as well as a brief description of the need for recycling phosphorous (in German).

Different strategies and methods to achieve the target of recovering phosphorus from sludge are discussed based on reports from the Swedish environment protection agency (SEPA) and with special emphasis on extraction of phosphate from wastewater, sludge and ash at central plants. Some possible improvements are briefly discussed, such as the use of certain comparatively clean sludge fractions and use of two-stage technology for the recovery.

Sludge Treatment and Disposal is the sixth volume in the series Biological Wastewater Treatment. The book covers in a clear and informative way the sludge characteristics, production, treatment (thickening, dewatering, stabilisation, pathogens removal) and disposal (land application for agricultural purposes, sanitary landfills, landfarming and other methods). Environmental and public health issues are also fully described.

This thesis proposes a new global goal – phosphorus security – to be integrated in the dominant research discourses and policy debates on global food security and global environmental change. Among other criteria, phosphorus security requires that phosphorus use is decoupled from environmental degradation and that farmers’ access to phosphorus is secured.

ASH DEC Umwelt AG, currently Outotec, operates manufacturing plants to recycle nutrients and metals from incineration residues. The focus of interest is the production of phosphorous-fertilisers and –raw products from sewage sludge ash with the patented ASH DEC technology.

Homepage of a the Sustainable and Safe Re-use of Municipal Sewage Sludge for Nutrient Recovery – SUSAN Project, a Specific Targeted Research Project (STREP) within the 6th framework of the European Union.